1. Field of the Invention
The present invention relates to an optical element having a reflective function and to a reflective display device including such an optical element.
2. Description of the Related Art
In recent years, a micro lens having an extremely small lens diameter and an array of such micro lenses have been developed and more and more extensively applied to the fields of optical communications and display devices. Along with those micro lenses and micro lens arrays, various other micro optical elements, including micro mirrors and micro prisms, have also been developed day after day. And it is expected that the optical technology and display technology will be further developed and advanced by realizing those micro optical elements.
A reflective liquid crystal display device, including a retroreflector as such a micro optical element, is disclosed in Japanese Laid-Open Publication Nos. 11-7008 and 2000-19490, for example. Using a retroreflector, an incoming light ray may be retro-reflected, or reflected along a path parallel to that of the incoming light ray. Accordingly, in the reflective liquid crystal display device, reflected part of light that has been emitted from a light source located near the user selectively reaches the user""s eyes but reflected part of other external light sources (e.g., illuminator or sun) does not reach his or her eyes. In this manner, unwanted back reflection (i.e., glare) is minimizable and the visibility is improvable. Also, since the reflective liquid crystal display device reduces the unwanted back reflection by using such a retroreflector, there is no need to reduce the intensity of the reflected light by intentionally decreasing the reflectance of the reflector, for example. As a result, display of a bright, high-contrast image is realized.
A retroreflector for use in the reflective liquid crystal display device, for example, may be formed as a micro optical element such as a corner cube array. A corner cube typically has three perpendicularly opposed reflective planes. The corner cube is an optical element for reflecting an incoming light ray back to its source by getting the light ray reflected by each one of those reflective planes after another The corner cube can always reflect the incoming light back to its source irrespective of its angle of incidence Hereinafter, a conventional reflective liquid crystal display device 80, including a retroreflector that has been formed as a corner cube array, will be described with reference to FIG. 1.
The reflective liquid crystal display device 80 includes: a substrate 82 on which a corner cube array 83 has been formed; a transparent substrate 81 located closer to an observer; and a polymer-dispersed liquid crystal layer 84 interposed between these substrates 81 and 82. A metallic reflective film 85 has been formed on the corner cube array 83. When color black should be displayed, incoming light, which has been transmitted through the transparent substrate 81 and the polymer-dispersed liquid crystal layer 84 controlled to a light transmitting state, can be reflected back toward its origin. The concave portions of the corner cube array 83 are filled with a transparent flattening member 86, on which a transparent electrode 87 has been formed. A color filter layer 88 and another transparent electrode 89 are provided on the surface of the transparent substrate 81 that is opposed to the liquid crystal layer 84. By regulating the voltage applied between the transparent electrodes 87 and 89, the reflective liquid crystal display device 80 controls the light transmittance (or scattering state) of the polymer-dispersed liquid crystal layer 84, thereby displaying an image thereon.
The size L1 of each corner cube included in the display device 80 is preferably equal to or smaller than the size L2 of each pixel. Accordingly, if the pixel size L2 of a display device is about 100 xcexcm, the corner cube size L1 is preferably several tens xcexcm or less. For example, Japanese Laid-Open Publication No. 11-7008 describes that when an array of quadrangular pyramidal concave portions is formed, the upper square of each quadrangular pyramidal concave portion should have a minimum size of about 5 xcexcm each side.
In the conventional reflective liquid crystal display devices, the retroreflector thereof often has triangular pyramidal, quadrangular pyramidal or spherical concave portions. However, there are only a limited number of optical element shapes that can be formed precisely enough at that small size of several tens xcexcm or less. An optical element including only concave or convex portions is relatively easy to shape. As for a display device that is currently having its pixel size reduced as much as possible to realize a high resolution, a micro corner cube should be formed at a very small size and with sufficiently high shape precision. Nevertheless, it is difficult to form such a micro corner cube in a complex shape.
On the other hand, it is known that a retroreflector of a relatively large size for use in a road sign, for example, includes corner cubes of a more complex shape. Hereinafter, a corner cube of such a complex shape will be described with reference to FIGS. 2A through 2C.
As shown in FIGS. 2A through 2C, the corner cube 90 has a structure including three substantially square reflective planes S1, S2 and S3 that are opposed almost perpendicularly to each other. As shown in FIG. 2C, an incoming light ray, which has been incident onto the corner cube 90, is reflected by one of these three planes S2, S3 and S1 after another, for example, so as to be reflected back to the direction from which it comes. In the corner cube 90, the substantially square reflective planes S1, S2 and S3 correspond to three of the six planes of a cube, which share one vertex of the cube. As shown in FIG. 2A, the corner cube 90 is made up of convex portions 92, each having a highest point indicated by an open circle ∘ (which is higher in level than intermediate points indicated by crosses X), and concave portions 94, each having a lowest point indicated by a solid circle xe2x97xaf (which is lower in level than the intermediate points indicated by the crosses X).
Such a corner cube 90 (which will be herein referred to as a xe2x80x9ccubic corner cubexe2x80x9d) has a shape including both the convex portions 92 and concave portions 94. Accordingly, compared to an optical element including only concave or convex portions (e.g., triangular pyramidal portions) as disclosed in Japanese Laid-Open Publication No. 11-7008, for example, it is more difficult to make this cubic corner cube 90. Hereinafter, a conventional method of making the cubic corner cube array shown in FIGS. 2A through 2C will be described.
In a pin bundling method, the end of a hexagonal columnar metal pin is provided with a prism having three square facets that are opposed perpendicularly to each other, and a number of such pins are bundled together to make a collection of prisms. In this manner, a cubic corner cube is made up of three facets of three prisms that are formed at the respective ends of three adjacent pins.
According to this method, however, a corner cube array should be made by collecting a plurality of prisms that have been separately formed for mutually different pins. Thus, it is actually difficult to make a corner cube of a small size. The minimum possible size of a corner cube (as indicated by L3 in FIG. 2B) that can be formed by this method is 1 mm. That is to say, a cubic corner cube having a size of several tens xcexcm is hard to make by this method.
In a plate method, a number of flat plates, each having two mutually parallel planes, are stacked one upon the other. At the side end face of these flat plates stacked, V-grooves are cut vertically to the parallel planes at an equal pitch, thereby forming a series of roof-shaped protrusions each having an apical angle of approximately 90 degrees. Next, each of these flat plates is horizontally shifted with respect to adjacent one of them so that the tops of the series of roof-shaped protrusions formed on the former plate are aligned with the bottoms of the V-grooves formed on the latter plate. In this manner, a die for use to make a cubic corner cube array is obtained.
According to this method, however, it is necessary to accurately shift and secure the flat plate having the roof-shaped protrusions with respect to the adjacent flat plate so that these two plates satisfy a required positional relationship. Thus, it is also difficult to make a cubic corner cube of as small a size as 100 xcexcm or less by this method.
A corner cube array for use as a retroreflector for a reflective liquid crystal display device needs to have a size equal to or smaller than a pixel size. Thus, in the prior art, the corner cube array has been formed to include only triangular pyramidal concave or convex portions that are relatively easy to make. However, if such a corner cube array including only those triangular pyramidal concave or convex portions (which will be herein referred to as a xe2x80x9ctriangular pyramidal corner cube arrayxe2x80x9d for convenience sake) is used, then the incoming light cannot be retro-reflected so efficiently as the cubic corner cube array. Hereinafter, it will be described with reference to FIGS. 3A through 3D how the incoming light is reflected by triangular pyramidal and cubic corner cubes.
FIGS. 3A and 3B illustrate a triangular pyramidal corner cube 96, while FIGS. 3C and 3D illustrate a cubic corner cube 98. As shown in FIG. 3B, a light ray A, which has been incident onto the center portion of the triangular pyramidal corner cube 96, is retro-reflected as indicated by the dashed line. But a light ray B, which has been incident onto an edge portion of the corner cube 96, is not retro-reflected. Accordingly, the triangular pyramidal corner cube 96 has non-retro-reflecting regions 96a at its three corner portions as shown in FIG. 3A. On the other hand, even the light ray B that has been incident onto an edge portion of the cubic corner cube 98 is also retro-reflected as shown in FIG. 3D. Thus, the cubic corner cube 98 has a broader retro-reflecting region on each reflective plane thereof and can appropriately retro-reflect a greater percentage of the incoming light.
The triangular pyramidal corner cube has non-retro-reflecting regions. Accordingly, if a retroreflector including those triangular pyramidal corner cubes is used for a reflective display device, part of the light that has been transmitted through the liquid crystal layer when color black should be displayed is sometimes not retro-reflected but reflected non-parallelly to the incoming light. Thus, part of the light that has been emitted from a distant, external light source may reach the user""s eyes. As a result, the contrast ratio may be decreased.
A problem like this has not been regarded as a serious one where a retroreflector is used mainly to prevent the unwanted projection of external light. This is because even a retroreflector including triangular pyramidal corner cubes can retro-reflect the incoming light from most parts thereof and can achieve the object of preventing the regularly reflected part of the external light from reaching the user""s eyes.
To minimize the decrease in contrast ratio for a reflective display device, the incoming light is preferably retro-reflected more efficiently by using a retroreflector including the cubic corner cubes. According to the conventional methods of making cubic corner cubes, however, it is possible to make cubic corner cubes of a relatively large size but it is virtually impossible to make cubic corner cubes of as small a size as 100 xcexcm or less. Thus, it has been very hard to use a cubic corner cube array as a retroreflector for a liquid crystal display device.
Likewise, as for a micro optical element other than the cubic corner cube array, it has also been extremely difficult to make the optical element in a complex shape, at a very small size and with sufficiently high shape precision.
In order to overcome the problems described above, the present invention provides (1) an optical element that performs a desired function and yet can be formed at a very small size and (2) a reflective display device including such an optical element.
An optical element according to the present invention includes first and second members. The first member has a first surface including a first concave portion. The second member has a second surf ace including a second concave portion and transmits incoming light therethrough. The first and second members are disposed so that the first and second surfaces are opposed to each other. First and second reflective regions have been formed on the first and second concave portions, respectively. At least part of the incoming light that has been transmitted through the second member is reflected from at least one of the first and second reflective regions.
In one preferred embodiment of the present invention, the first surface includes the first concave portion and a flat portion, the second surface includes the second concave portion and a flat portion, and the first and second concave portions are so disposed as not to face each other.
In another preferred embodiment of the present invention, the first and second concave portions have substantially the same shape.
In still another preferred embodiment, each of the first and second concave portions has a triangular pyramidal shape, and the first and second concave portions constitute a part of a corner cube.
In this particular preferred embodiment, at least part of the incoming light that has been transmitted through the second member is reflected from both of the first and second reflective regions so that the incoming light is retro-reflected.
In yet another preferred embodiment, at least one of the first and second reflective regions is made of a metal film.
In yet another preferred embodiment, the second reflective region is made of a material that has a refractive index lower than that of the second member.
In this particular preferred embodiment, the optical element further includes a member for filling the first concave portion on the first reflective region. The first reflective region is made of a material that has a refractive index lower than that of the member for filling the first concave portion.
A reflective display device according to the present invention includes: the optical element according to any of the preferred embodiments of the present invention described above; and a light modulating layer interposed between the first and second members.
In one preferred embodiment of the present invention, the light modulating layer includes a scattering-type liquid crystal layer.
In this particular preferred embodiment, the reflective display device may further include: a first flattening member that fills the first concave portion of the first member; and a second flattening member that fills the second concave portion of the second member. The scattering-type liquid crystal layer is interposed between the surface of the first member that has been flattened by the first flattening member and the surface of the second member that has been flattened by the second flattening member.
In another preferred embodiment of the present invention, the scattering-type liquid crystal layer fills the first concave portion of the first member.
Another reflective display device according to the present invention includes: the optical element according to any of the preferred embodiments of the present invention described above; a transparent substrate disposed to face the optical element; and a light modulating layer, which is interposed between the optical element and the transparent substrate and controlled to assume either a light scattering state or a light transmitting state.
The present invention provides an array of corner cubes, each having three facets that are opposed substantially perpendicularly to each other. Each of the three facets of each said corner cube includes: a first surface of a concave portion that has been formed in a member; and a second surface of a convex member that has been formed on the member so as to be adjacent to the concave portion.
In one preferred embodiment of the present invention, the member includes the concave portion and a flat portion adjacent to the concave portion, and the convex member has been secured onto the flat portion.
In another preferred embodiment of the present invention, the first surface of the concave portion and the second surface of the convex member each have a planar shape of a rectangular isosceles triangle, and each of the three facets of each said corner cube is substantially square.
An inventive method of making an optical element includes the steps of: forming a first concave portion in a first member and forming a first reflective region on the first concave portion; forming a second concave portion in a second member and forming a second reflective region on the second concave portion; and disposing the first and second members in such a manner that a surface of the first member in which the first concave portion has been formed is opposed to a surface of the second member in which the second concave portion has been formed.
An inventive method of making a corner cube array includes the step of a) preparing a first member in which at least one first concave portion has been formed in a triangular pyramidal shape. The first concave portion is made up of three triangular facets that are opposed substantially perpendicularly to each other. The method further includes the step of b) preparing a second member in which at least one second concave portion has been formed in the triangular pyramidal shape. The second concave portion is made up of three triangular facets that are opposed substantially perpendicularly to each other. The method further includes the step of c) disposing the first and second members in such a manner that a surface of the first member in which the first concave portion has been formed is opposed to a surface of the second member in which the second concave portion has been formed. The corner cube array is made up of a plurality of cubic corner cubes, each of which includes a first set of triangular planes defined by the first concave portion and a second set of triangular planes defined by the second concave portion.
In one preferred embodiment of the present invention, the method further includes the steps of: forming a reflective region on each of the three triangular facets of the first concave portion; and forming a reflective region on each of the three triangular facets of the second concave portion. The second member is transparent. The reflective regions provided for the first concave portion and the reflective regions provided for the second concave portion are arranged substantially continuously to each other so that when the reflective regions provided for the first concave portion are used as concave reflective regions, the reflective regions provided for the second concave portion are used as convex reflective regions.
In another preferred embodiment of the present invention, the method further includes the steps of: filling the triangular pyramidal first concave portion of the first member with a convex member having a triangular pyramidal shape corresponding to that of the first concave portion before the step c) is performed; and securing the convex member in the triangular pyramidal shape onto on the second member after the step c) has been performed.
Another inventive method of making a corner cube array includes the steps of: a) preparing a member including a plurality of triangular pyramidal concave portions, each of which has three perpendicularly opposed equilateral triangular facets, in a predetermined surface thereof; and b) forming a plurality of triangular pyramidal convex members, each of which has three perpendicularly opposed equilateral triangular facets, on the predetermined surface of the member.
In one preferred embodiment of the present invention, the step b) includes the steps of: disposing a retaining member on the predetermined surface of the member to retain the convex members thereon; and transferring the convex members from the retaining member onto the predetermined surface of the member.
Still another inventive method of making a corner cube array includes the steps of: a) preparing a member including a plurality of triangular pyramidal concave portions, each of which has three perpendicularly opposed equilateral triangular facets; and b) forming a plurality of triangular pyramidal convex members, each of which has three perpendicularly opposed equilateral triangular facets, on a predetermined surface of the member.
In one preferred embodiment of the present invention, the step b) includes the steps of: disposing a retaining member on the predetermined surface of the member to retain the convex members thereon; and leaving the convex members on the predetermined surface of the member by dissolving the retaining member.
In another preferred embodiment, the step a) includes the steps of: forming grooves in three directions of a base material; transferring unevenness of the base material, in which the grooves have been formed, to a transfer material; and filling every other one of concave portions that have been formed in the transfer material.
In still another preferred embodiment, the step a) includes the step of anisotropically etching {111} planes of a cubic single crystalline substrate.
In yet another preferred embodiment, the step a) includes the step of pressing a pin, which has a triangular pyramidal convex member made up of three perpendicularly opposed equilateral triangular facets, onto a base material.
A die according to the present invention is used to make a micro corner cube array includes a base member including a plurality of triangular pyramidal concave portions, each of which has three perpendicularly opposed equilateral triangular facets, in a predetermined surface thereof; and a plurality of triangular pyramidal convex members, each of which has three perpendicularly opposed equilateral triangular facets, formed on the predetermined surface of the base member.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.